HTTP: The Web's Foundation

My reading note about book "HTTP - The Definite Guide", part I: The Web's Foundation. After reading this article, you'll understand the definition of HTTP, URL and resources, the flow of messages, ...

It’s been some days that I read the book HTTP - The Definite Guide. Today, I want to share with you what I’ve learn from Part I. The Web’s Foundation. After reading this blog, you’ll understand:

  • The Definition of HTTP
  • URL and Resources
  • The Flow of Messages
  • Different Status Codes HTTP

Definition of HTTP

HTTP, the Hypertext Transfer Protocol, is the common language of the modern global internet. It transports the Web’s traffic between server and clients. When using HTTP, you don’t have to worry about the destroyed, duplicated, distorted in transit. A common client of HTTP is browser. When browsing a web page, the browser sends a HTTP request to the server, and server gives a HTTP response, along with the type of the object (MIME type[1]), the length of the object, and other information.

URL and Resources

URL, the uniform resource locator, provides a means of locating any resource on the Internet, but these resources can be accessed by different schemes (e.g., HTTP, FTP, SMTP), and URL syntax varies from scheme to scheme. Here’s an example:
  1. The 1st part of the URL is scheme, which describes the protocol to use to access the resource. This is usually http or https.
  2. The 2nd part is the server address, which can be a host name or an IP address. If the port number is not defined, the default port is 80.
  3. The 3rd part is the resource of the web server.

Schemes describe what protocol to use. The scheme http and https are twins: the only difference is that the https scheme uses Netscape’s Secure Sockets Layer (SSL), which provides end-to-end encryption of HTTP connections. Its syntax is identical to that of HTTP.

The Flow of Messages

HTTP messages are the blocks of data sent between HTTP applications. These blocks of data begin with some text meta-information describing the message contents and meaning, followed by optional data. These messages flow between clients, servers, and proxies. The terms “inbound”, “outbound”, “upstream”, and “downstream” describe message direction.

HTTP uses the terms inbound and outbound to describe transactional direction. Messages travel inbound to the origin server, and when their work is done, they travel outbound back to the user agent (see the figure below).

Figure 3-1. Messages travel inbound to the orgin server and outbound back to the client

All HTTP messages fall into two types: request messages and response messages. Request messages request an action from a web server. Response messages carry results of a request back to a client. Both request and response messages have the same basic structure.

Here’s the format for a request message:

<method> <request-URL> <version>


Here’s the format fot a response message:

<version> <status> <reason-phrase>


Note that the only difference is in the start line.

Status Codes

HTTP status codes are classified into five broad categories.

  • 1xx Informational responses An informational response indicates that the request was received and understood.
  • 2xx Success This class of status codes indicates the action requested by the client was received, understood, accepted, and processed successfully.
  • 3xx Redirection This class of status code indicates the client must take additional action to complete the request. Many of these status codes are used in URL redirection.
  • 4xx Client errors The 4xx class of status codes is intended for situations in which the client seems to have erred.
  • 5xx Server error The server failed to fulfil an apparently valid request. It indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request.

If you think they’re too complex, then please see this excellent Tweet:


[1] MIME type: type of Multipurpose Internet Mail Extensions. Media types were originally defined in Request in November 1996 as a part of MIME specification, for denoting type of email message content and attachments.